MANY STUDIES HAVE DEMONSTRATED THE PROFOUND EFFECTS THAT HIGH FAT DIETS HAVE ON PLASMA LIPID CONCENTRATIONS. IN OUR STUDIES WE HAVE EXAMINED THE EFFECTS OF HIGH FAT HIGH CHOLESTEROL DIETS ON APOB KINETIC PARAMETERS IN THE BABOON. THE FINDINGS OF THIS STUDY HAVE RECENTLY BEEN PUBLISHED. BRIEFLY, THESE STUDIES SUGGEST THAT THE VARIABILITY OF PLASMA LDL CHOLESTEROL ON THE CHOW DIET IS DUE THE VARIABILITY IN LDL APOB CATABOLIC RATE AND THE HEPATIC LDL RECEPTOR TRANSCRIPTION. HOWEVER, THE VARIABILITY IN LDL CHOLESTEROL ON THE HIGH FAT HIGH CHOLESTEROL DIET IS REGULATED BY LDL PRODUCTION, WHICH IS NOT CORRELATED WITH HEPATIC LDL RECEPTOR OR APOB MRNA LEVELS.IN HUMANS WE ARE INVESTIGATING THE EFFECTS OF THE AHA STEP I AND STEP II DIETS ON APOB METABOLISM. ALTHOUGH OTHER GROUPS HAVE DEMONSTRATED THE BENEFICIAL EFFECTS OF THESE DIETS ON PLASMA LIPIDS NO STUDIES INVESTIGATING THE MECHANISM/S OF THEIR ACTION HAVE BEEN UNDERTAKEN. THESE STUDIES ARE IN PROGRESS.